1. Field of the Invention
The invention relates to a reverse conducting IGBT (Reverse Conducting Insulated Gate Bipolar Transistor).
2. Description of Related Art
A reverse conducting IGBT that has an IGBT structure and in which a diode structure is integrated into a semiconductor layer has been developed. As shown in FIG. 5, this kind of reverse conducting IGBT is often used for six transistors Tr1 to Tr6 that form a three phase inverter. The diode structure operates as a Free Wheeling Diode (FWD).
FIG. 6 is a view of a timing chart of a gate signal input to each of the transistors Tr1 to Tr6. The phases of the gate signals of the upper and lower transistors are offset approximately 180 degrees, the phases of the gate signals of the transistors of a U-phase and a V-phase are offset approximately 120 degrees, and the phases of the gate signals of the transistors of the V-phase and a W-phase are offset approximately 120 degrees.
For example, at time t1, current flows through the IGBT structure of each of the transistors Tr1, Tr3, and Tr6. At time t3, current flows through the IGBT structure of each of the transistors Tr2, Tr3, and Tr6. Here, at time t2 that is a transition from time t1 to time t3, return current flows through the diode structure of the transistor Tr2, and the gate signal of the transistor Tr2 is on. In this way, with a three phase inverter, when return current flows through the diode structure, there is a mode in which gate voltage is applied to a corresponding gate.
When gate voltage is applied to the gate, an n-type drift region and an n-type emitter region are connected via a channel. Both the n-type emitter region and a p-type body region are connected to a common emitter electrode. Therefore, when gate voltage is applied to the gate, it becomes difficult for sufficient voltage to be applied in the forward direction of the diode structure formed by the p-type body region and the n-type drift region. This phenomenon is referred to as gate interference, and forward voltage of the diode structure fluctuates largely depending on whether the gate signal is on or off, as shown in FIG. 7.
To counter such gate interference, Japanese Patent Application Publication No. 2008-72848 (JP 2008-72848 A) proposes technology that detects return current flowing through the diode structure, and provides an external circuit configured to interrupt the gate signal.
However, providing an external circuit as described in JP 2008-72848 A complicates control and increases costs.